There has been a great deal written about various optical temperature measuring techniques, both in patents and the technical literature, as well as many commercial products utilizing this technology. In one aspect of this technology, a luminescent material is used as a temperature sensor because certain aspects of its luminescence are temperature dependent. Typically in the form of a sensor at the end of a fiber optic cable, the luminescent material is excited to luminescence by sending excitation radiation of one wavelength to the sensor through the optical fiber, and the resulting luminescence at a different wavelength is photo-detected after passing back along the optical fiber. The detected signal is then processed to determine the temperature of the luminescent material in the sensor. Basic concepts of luminescent temperature sensing, as well as many different forms of sensors, are described in U.S. Pat. No. 4,448,547. The measurement of the decay time of the luminescence after termination of an excitation pulse, as a measurement of temperature, is described in U.S. Pat. No. 4,652,143. Commercial products adopted the decay time measurement technique as a good measurement of temperature. One advantage and focus of luminescent temperature measurement techniques has been for applications in environments having strong electric and/or magnetic fields and the like, where metal sensors cannot be relied upon to provide accurate results because the metal is heated when immersed in the electromagnetic field, causing a bias in the readings.
Applications of these luminescent sensor measurement techniques are numerous, including the measurement of surface temperature. U.S. Pat. No. 4,752,141 describes an elastomeric luminescent sensor at the end of an optical fiber that deforms as it is pushed against a surface being measured in order to establish good thermal contact. Another embodiment employing a thin non-metallic disc with a layer of luminescent material between it and the end of an optical fiber is also described.
Another optical temperature measuring technique relies upon the infrared emissions of a black-body sensor, or one having the characteristics of a black-body. An example of such a system, generally used to measure higher temperatures than measured with luminescent sensors, is described in U.S. Pat. No. 4,750,139. The sensor is a black-body emitter formed at the end of an optical fiber. U.S. Pat. No. 5,183,338 describes several forms of a fiber optic sensor that includes both luminescent and blackbody temperature measuring elements. Each of the foregoing identified patents is expressly incorporated herein in its entirety by this reference.
There are also many other optical temperature sensing techniques that have been described in patents and the literature, as well as being used commercially. But the luminescent and black-body techniques have generally been preferred over those others.